A Preliminary Study of Zirkite Ore 

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A THESIS 

PRESENTED TO TH& FACULTY OF THE GRADUATE SCHOOL 
OF CORNELL UNIVERSITY FOR THE DEGREE OF 
DOCTOR OF PHILOSOPHY 


By 

JOHN GRAHAM THOMPSON 



(Reprinted from the Journal of Physical Chemistry, 
Vol. XXVI, No. 9, December, 1922.) 








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A Preliminary Study of Zirkite Ore 


A THESIS 

PRESENTED TO THE FACULTY OF THE GRADUATE SCHOOL 
OF CORNELL UNIVERSITY FOR THE DEGREE OF 
DOCTOR OF PHILOSOPHY 


By 

JOHN GRAHAM THOMPSON 


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(Reprinted from the Journal of Physical Chemistry, 
Vol. XXVI, No. 9, December, 1922.) 













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A PRELIMINARY STUDY OF ZIRKITE ORE 


BY J. G. THOMPSON 

This investigation consists of a study of zirkite ore (crude 
zirconium oxide), with attempts to improve the refractory 
properties by the elimination of certain of the impurities 
present in the raw material. 

Pure zirconium oxide, if available in sufficient quantity, 
would aid materially in solving the problems of the users of 
high-temperature refractories, particularly those problems con¬ 
nected with the extreme temperatures encountered in the use of 
the electric furnace. As a refractory for high temperatures 
zirconium oxide is far superior to any of the materials commonly 
employed for this purpose and in some respects it approaches 
the ideal refractory. It possesses 1 a melting point higher than 
that of any other known metallic oxide; it is non-volatile 
below its melting point; it is neutral in character and inert to 
the action of practically all of the common reagents; it pos¬ 
sesses low coefficients of thermal and electrical conductivitv. 

J 

These properties ensure the successful use of pure zirconium 
oxide as a refractory, even for the most extreme service con¬ 
ditions, at any time when it becomes available in quantity. 

Zirconium was discovered by Klaproth in 1789; but, for 
many years after its discovery, it was considered one of the 
rare earths, and occasional samples of zircon, the orthosilicate of 
zirconium, were the only sources of supply. At present depos¬ 
its of zircon sands, usually associated with monazite sands, 
are known to exist in many localities but the deposits usually 
are of limited extent and the zircon content is small. Zirco¬ 
nium also occurs in small amounts in a number of rare ores but 
these are important only for the traces of radio-active elements 

1 Meyer: Met. Chem. Eng., 12 , 791 (1914); 13 , 263 (1915); Hedvall: 
Zeit. anorg. Chem., 93 , 313 (1915); Bradford: Chem. Trade Jour., 62 , 284 (1918); 
Arnold: Jour. Soc. Chem. Ind., 37 , 724 (1918); Granger: Chem. News, 118 , 
115, 121 (1919). 





813 


J. G. Thompson 


which they contain. Present interest centers solely upon the 
oxide ores of zirconium generally known as “Baddeleyite” 
ores. This group includes a number of ores of indefinite and 
variable composition in which the zirconium oxide content 
varies up to 99 percent. The name “Zirkite” has been re¬ 
stricted to the baddeleyite ore from the huge Brazilian deposits 
which now supply practically all of the demand for zirconium. 
Zirkite ore is variable in composition but careful sorting at the 
mine furnishes a product which is fairly uniform in composi¬ 
tion averaging about 80 percent zirconium oxide. It is stated 1 
that an unlimited supply of this ore is available as the 
deposits are of vast extent. The only factors limiting the output 
are the difficulties due to primitive methods of mining and 
transportation. 

The use of zirconium oxide always has depended upon its 
refractory properties; but this use, until recently, was confined 
to the small amounts of very pure material required for the 
incandescent element in the Nernst, Drummond, and Bleriot 
lamps; as an additional element mixed with the thoria and 
ceria in Welsbach mantles; etc. The discovery of the huge 
Brazilian deposits of high-grade ore has awakened much inter¬ 
est in the possibility of using this ore on a large scale as a 
refractory. 

Many of the properties of the raw ore 2 are comparable 
with those of the pure oxide but the melting point (2950°- 
3000° C for pure Zr0 2 ) drops to 1200°-2000° C for the raw 
ore, depending upon the composition. Zirkite is an excellent 
refractory within the limits set by its softening and melting 
points but the low melting point obviously detracts greatly 
from the refractory value of the ore. It has been used suc¬ 
cessfully as raw material in the manufacture of combustion 
boats, tubes, etc., for high temperature work in the laboratory; 

1 Meyer: Foote Mineral Company publication for November, 1916. 

2 Meyer: Met. Chem. Eng., 12, 791 (1914); Audley: Trans, Eng. Ceram. 
Soc., 16 , 121 (1917); Rosenhain: Trans. Faraday Soc., 12 , 178 (1917); Devereux: 
Met. Ind., 16 , 414 (1920). 



A Preliminary Study of Zirkite Ore 


814 


but the successful use of the material on a large scale awaits the 
development of means of raising the melting point. 

The problem obviously is the elimination of the element 
or elements which exert so detrimental an effect upon the 
melting point of the ore. No definite information is available 
regarding the specific effect of the various impurities upon the 
melting point of pure zirconium oxide, but silicon and iron, 
which together constitute the bulk of the impurities, naturally 
are regarded as the chief sources of trouble. 

Methods of purification, as recorded in the literature, 
may be divided into two classes: 

(1) Methods for the preparation of pure zirconium com¬ 
pounds. 1 

(2) Methods for partial purification of the ore by elimi¬ 
nation of impurities, either by leaching 2 or by volatiliza¬ 
tion. 3 

Those methods in the first class which are successful are 
adaptable only for the production in the laboratory of small 
amounts of pure zirconium compounds. The methods in 
class two have been confined almost entirely to the removal of 
iron. The refractory silicates of zirconium were not affected 
at the relatively low temperatures used in Phillips’ chlorina¬ 
tion experiments and are not susceptible to leaching unless 
they have been decomposed by a preliminary fusion. 4 The 
methods in this class are better adapted for large scale operation 
but are successful only to a limited extent in producing satis¬ 
factory refractory material from the raw ore. 

1 Berlin: Jour, prakt. Chem., 58 , 147 (1853); Hermann: Ibid., 97, 330, 
340 (1866); Bailey: Chem. News, 53 , 55, 260, 287 (1886); Doremus: Jour. 
Am. Chem. Soc., 8 , 91 (1886); Bayer: Zeit. angew. Chem., 23 , 485 (1910); 
Anonymous: Ceramique, 14,204 (1911); Loveman: U. S. Patent No. 1,261,984 
(1918); Imray: British Patent No. 16,555 (1913); Leuchs: German Patent No. 
285,344 (1914); Meyer: loc. cit., and Granger: loc. cit. 

2 Wedekind: Zeit. angew. Chem., 21 , 2270 (1908); Weiss and Lehmann: 
Zeit. anorg. Chem., 65 , 178 (1909); Wedekind: Ber. deutsch. chem. Ges., 43,290 
(1910); Ramsden: Met. Ind., 16 , 3 (1920); Audley; Granger; Rosen: loc. cit. 

3 Phillips: Jour. Am. Ceramic Soc., 1, 791 (1918). 

4 Weiss and Lehmann: loc. cit., Jost and Plocker: German Patent No. 

285,981 (1914). ’ 



815 


J. G. Thompson 


The remarkable stability at high temperatures of certain 
compounds of zirconium, particularly the oxide and carbide, 
together with the fact that these compounds are not appreci¬ 
ably volatile below their decomposition points, suggested the 
use of electric heating as a means of purifying zirkite ore by 
the elimination of impurities whose oxides and carbides pre¬ 
sumably are less refractory than the corresponding zirconium 
compounds. Hitherto the electric furnace has been employed 1 
only as a preliminary step in the purification of zirkite, appar¬ 
ently for the sole purpose of decomposing refractory silicates. 
This investigation, therefore, was undertaken for the sake of 
ascertaining to what extent zirkite might be freed from im¬ 
purities by direct heating in an electric furnace. 

Before the experimental portion of this investigation 
could be undertaken, a scheme of analysis had to be outlined. 
A search of the literature revealed many conflicting statements 
and considerable time was devoted to the investigation of 
the various methods before a satisfactory scheme was obtained. 
A brief, critical review of some of the methods proposed, 
together with a description of the scheme adopted, is therefore 
included in this report. 

The Analysis of Zirconium Compounds 

The characteristic properties of zirconium ores which 
make them valuable, i. e., the refractory properties and chem¬ 
ical inertness, formed the first obstacles to be overcome in the 
decomposition of the ore. 

Four general methods of attack have been proposed: 

(1) Fusion with hydrofluoric acid or with fluorides 2 

(2) Conversion to carbides in the electric furnace 3 

1 Moissan: Comptes rendus, 116 , 1222 (1893); Troost: Ibid., 116 , 1428 
(1893); Podszus: Jour. Soc. Chem. Ind., 36 , 217 (1917); Barton: U. S. Patent 
No. 1,342,084 (1920); Weiss and Lehmann: loc. cit. 

2 Marignac: Ann. Chim. Phys., (3) 60 , 260 (1860); Weiss and Neumann: 
Zeit. anorg. Chem., 65 , 248 (1909); Anonymous: The Brass World, 7, 46 (1911); 
Meyer: Foote Mineral Company Publication for November 1916; Weiss and 
Lehmann: loc. cit. 

3 Granger; Moissan; Podszus; Troost; Weiss and Lehman: loc. cit. 



A Preliminary Study of Zirkite Ore 


816 


(3) Fusion with sulphuric acid or its derivatives 1 

(4) Fusion with alkalies 2 

Fusion with sodium tetraborate (borax) was found to fur¬ 
nish the best means for the decomposition of the materials en¬ 
countered in this investigation. Borax fusions usually pro¬ 
duce complete decomposition in one operation but the boric 
acid must be eliminated thereafter if a complete analysis is de¬ 
sired. Fusion with alkalies produces decomposition of the 
silicates and phosphates but is not applicable to oxide ores. 
Fusion with pvrosulphates decomposes oxide ores but is not ap¬ 
plicable to silicates. Methods involving fusion with fluorides 
or conversion to carbides in the electric furnaces are objection¬ 
able on account of loss through volatilization. A combination 
of alkali fusions, followed or preceded by pyrosulphate 
fusions, eventually produces complete decomposition and 
total solution of any zirconium compound, but the pro¬ 
cedure is tedious and involved. In all cases the method em¬ 
ployed for decomposition is determined by the nature and 
characteristics of the material. 

After the ore has been decomposed by fusion methods and 
brought into complete solution, the problem of the separation 
of the various elements arises. 

Analytical Separations 

Silicon is removed from the solution without difficulty 
owing to the insolubility in acids of its dehydrated oxide. 

Iron interferes with almost all of the methods for the 
precipitation of zirconium from solution, and should be re¬ 
moved before such precipitation is attempted. Leaching of 

1 Baskerville: Jour. Am. Chem. Soc., 16, 475 (1894); Dittrich and Freund: 
Zeit. anorg. Chem., 56, 337 (1907); Weiss: Ibid., 67, 456 (1910); Wedekind: 
Ber. deutsch. chem. Ges., 43, 290 (1910); Johnstone: “The Rare Earth Industry” 
(1915); Powell and Schoeller: Analyst, 44, 397 (1919); Weiss and Lehmann; 
Bayer: loc. cit. 

2 Jewett: “Bibliography of Zirconium” (1893); Dennis and Spencer: 
Jour. Am. Chem. Soc., 18, 674 (1896); Abegg: Handbuch, IV, 490 (1913); 
Schiotz: Chem. Abstracts, 12, 661 (1918); Travers: Chim. Ind., 2, 385 (1919); 
Lundell and Knowles: Jour. Am. Chem. Soc., 42, 1439 (1920); Bayer; Berlin; 
Johnstone; Powell and Schoeller: loc. cit. 




817 


J. G. Thompson 


freshly precipitated or ignited oxides by acid solutions, in¬ 
cluding oxalic and sulphurous acids 1 is not successful. 2 Smith 3 
claims to have effected complete separation of iron and zirco¬ 
nium by electrolytic methods, using a mercury cathode, and 
Price 4 records the formation of soluble perzirkonates by 
methods which would leave the iron insoluble. Neither of 
these results could be duplicated during this investigation. 

Only two really successful methods have been devised for 
the separation of iron from solutions containing zirconium. 
These are based, respectively, uppn selective solubility of ferric 
chloride in ether, 5 and the fact that iron is and zirconium 
is not precipitated bv hydrogen sulphide from ammoniacal 
solutions containing tartaric acid. 6 The ether separation 
method is the better of the two for the removal of large amounts 
of iron such as are encountered in the analysis of zirconium 
steels or ferro-zirconium. The precipitation method is not as 
satisfactory for the removal of large amounts of iron, owing to 
the difficulties encountered in handling large volumes of precipi¬ 
tated ferrous sulphide. It is indispensable however, for the 
removal of the traces of iron which almost invariably survive the 
ether separation, and is adaptable for the analysis of zirconium 
ores in which the iron oxide content does not greatly exceed 
five percent. 

A few attempts to precipitate zirconium and titanium 
separately are recorded in the literature, 7 but the success of 
some of these attempts has been questioned. 8 For analytical 

1 Dubois and Silveira: Ann. Chim. Phys., (1) 14, 110 (1820); Berthier: 
Ibid., (2) 50, 362 (1832); Wunder and Jeanneret: Zeit. analyt. Chem., 50, 733 
(1911). 

2 Berlin; Hermann: loc. cit. 

3 Smith: “Electro-Analysis” (1911). 

4 Price: “Per-Acids and Their Salts” (1912). 

5 Noyes, Bray and Spear: Jour. Am. Chem. Soc., 30, 481 (1908). 

6 Wedekind: loc. cit. 

7 Bailey: Jour. Chem. Soc., 49, 149, 481 (1886); Crookes: “Select 
Methods in Chemical Analysis” (1894); Dittrich and Freund: loc. cit.; Brown¬ 
ing, Simpson and Porter: Am. Jour. Science, 42, 106 (1916); Headden: Chem. 
Abstracts, 11, 2311 (1917). 

8 Mathews: Jour. Am. Chem. Soc., 20, 815 (1898); Weiss and Lehmann: 
loc. cit. 



A Preliminary Study of Zirkite Ore 


818 


purposes the separation of zirconium from titanium is un¬ 
necessary as titanium is readily determined by Weller’s 1 
colorimetric method using hydrogen peroxide. The presence of 
zirconium does not interfere with this determination. 

A variety of methods have been proposed for the simul¬ 
taneous precipitation of zirconium and titanium. Dittrich 
and Freund 2 claim that sodium acetate will produce complete 
precipitation of zirconium and titanium. The use of sulphur¬ 
ous acid or its derivatives has been proposed, 3 but Johnstone 4 
says that this reagent also precipitates some of the rare earths 
as well as traces of iron and aluminum and Hermann 5 reports 
that the separation resulting from the use of sulphur dioxide is 
not satisfactory. A number of these separations were at¬ 
tempted during the present investigation with complete 
lack of success. The manipulation is tedious, precipitation sel¬ 
dom is complete, and the precipitate usually is badly contami¬ 
nated. Precipitation of zirconium and titanium by weak 
organic bases has been recommended, 6 and a number of these 
precipitations were attempted, using phenylhvdrazine as the 
most promising member of this class. The experiments uni¬ 
formly were unsuccessful, the precipitation was slow and incom¬ 
plete, and the precipitate usually was contaminated. Varia¬ 
tions of Hillebrand’s method for the precipitation of zirconium 
phosphate 7 have been recommended. Lundell and Knowles 8 

1 Weller: Ber. deutsch. chem. Ges., 15, 2599 (1882). 

2 Dittrich and Freund: loc. cit. 

3 Berthier: Ann. Chitn. Phys., (2) 50, 362 (1832); (3) 7, 84 (1843); 
Chancel: Jour, prakt. Chem., 74, 471 (1858); Imray: Jour. Soc. Chem. Ind., 
9, 941 (1890); Trautmann: Zeit. angew. Chem., 24, 62 (1911); Baskerville; 
Dittrich and Freund; Weiss and Lehmann; Travers; Granger; Powell and 
Schoeller: loc. cit. 

4 Johnstone: loc. cit. 

5 Hermann: Ibid. 

6 Hess and Campbell: Jour. Am. Chem. Soc., 21, 776 (1899); Jefferson: 
Ibid., 24, 540 (1902); Allen: Ibid., 25,421 (1903); Hartwell: Ibid., 25,1128 (1903). 

7 Hillebrand: U. S. Geol. Survey, Bull. No. 148; Biltz and Mecklenburg: 
Zeit. angew. Chem., 25, 2110 (1912); Ferguson: Eng. Min. Jour., 106, 356, 793 
(1918); Steiger: Jour. Wash. Acad. Sci., 8, 637 (1918); Nicolardot and Reglade: 
Comptes rendus, 168, 348 (1919); Browning, Simpson and Porter: loc. cit.; Schiotz: 
loc. cit. 

8 Lundell and Knowles: Jour. Am. Chem. Soc., 41, 1801 (1919). 



819 


J. G. Thompson 


discuss the method in detail and point out that the variations 
in composition of the precipitate prevent the use of the method 
except for very small amounts of zirconium. The most 
satisfactory method for the analytical determination of zir¬ 
conium and titanium was found to be precipitation by cupferron, 
the ammonium salt of nitrosophenylhydroxylamine. The 
use of this reagent has been proposed by several authors 1 
and has been recommended recently by Lundell and Knowles 2 
who discuss the method in detail, including a list of the elements 
which interfere with the precipitation and a discussion of the 
precautions which must be observed. 

The filtrate from the cupferron precipitation contains 
the aluminum, rare earths, and traces of other elements. 
The scheme of separation for the elements in this filtrate de¬ 
pends upon the nature and number of elements present, as 
determined in the preliminary qualitative examination. 

From the foregoing data a complete scheme of analysis, 
adaptable to low-phosphorus baddeleyite ore, was evolved. 
In view of the time and labor required for a complete analysis, 
a modified scheme was adopted for the numerous analyses 
which attended the progress of this investigation. This 
modified scheme allows only the determination of silicon, iron, 
titanium, and zirconium, as follows: 

Modified Scheme fop Routine Analysis 

The sample is ground to 80-100 mesh. 0.5 gram of the 
sample is weighed out and fused 3 in platinum with 5 grams 
of borax until a clear, fused mass results. When fusion is 
complete (usually requiring about one-half hour at the full 
heat of a Meker burner) the crucible is removed from the 


1 Ferrari: Chem. Abstracts, 9, 1019 (1915); Thornton and Hayden: 
Am. Jour. Sci., 38, 137 (1914); Thornton: Ibid., 42, 151 (1916); Brown: Jour. 
Am. Chem. Soc., 39, 2358 (1917). 

2 Lundell and Knowles: Jour. Ind. Eng. Chem., 12, 344 (1920). 

3 The usual practice is to dehydrate the borax in the crucible, add the 
weighed sample, and proceed with the fusion. Carbide samples should be ignited 
to the oxides before attempting the fusion. 



A Preliminary Study of Zirkite Ore 


820 


flame. While solidification of the melt is taking place, the 
crucible is kept in motion so that when cold the fusion is dis¬ 
tributed around the sides of the crucible. The cooled melt is 
dissolved in 5 percent hydrochloric acid in a porcelain casserole, 
solution being hastened by gentle warming. When solution 
is complete, the crucible is removed and rinsed out, 10-15 cc 
concentrated sulphuric acid are added, and the solution is 
evaporated on a hot plate until fumes of sulphur trioxide ap¬ 
pear. The final fuming is done over a free flame, agitating the 
solution to prevent spattering. The residue is cooled, dis¬ 
solved by diluting with water, and filtered. The precipitate 
on the filter is washed with hot water, ignited, weighed, 
treated with hydrofluoric and sulphuric acids, again ignited and 
weighed, and the loss of silica determined. A slight residue, 
mainly iron, is recovered by fusion with pyrosulphate and 
added to the original filtrate. 

The filtrate from the removal of silica is diluted to about 
500 cc, precipitated with ammonia, and boiled to coagulate 
the precipitate. After settling, the supernatant liquid is de¬ 
canted as completely as possible to remove the bulk of the 
alkalis and boron. The ammonia precipitate is redissolved 
by adding 5-10 cc of concentrated sulphuric acid. One 
gram of tartaric acid is added and the solution is made dis¬ 
tinctly ammoniacal. The volume of the solution at this 
point should be 150-200 cc. The solution is treated with hy¬ 
drogen sulphide for twenty minutes, boiled for 3-4 minutes, 
allowed to cool, and filtered. The precipitate is covered and 
allowed to drain thoroughly but is not washed, owing to the 
decided tendency of the ferrous sulphide to pass through the 
filter when any washing solution is used. The filtrate is sub¬ 
jected to a second precipitation with hydrogen sulphide and is 
again boiled and filtered. The two precipitates of ferrous sul¬ 
phide are redissolved in dilute aqua regia and the iron is de¬ 
termined gravimetrically. The presence of hydrochloric acid 
and small amounts of organic matter (from the tartaric 
acid) precludes the determination of iron by volumetric 
methods. 


821 


J. G. Thompson 


The filtrate from the removal of ferrous sulphide, boiled to 
remove most of the hydrogen sulphide, is made acid with a 
known excess of sulphuric acid, boiled again, and the precipi¬ 
tated sulphur filtered off and discarded. The filtrate is cooled, 
diluted to 600 cc containing 7.5-10 percent free sulphuric acid, 
chilled in ice water, and precipitated by the addition of a cold 
6 percent aqueous solution of cupferron. During this precipi¬ 
tation the temperature must not exceed 15° C. The pre¬ 
cipitate is allowed to stand one-half hour and is then filtered 
using gentle suction and washing with cold, 5 percent hydro¬ 
chloric acid. The filtrate and washings are discarded. The 
precipitate is ignited and weighed as Zr0 2 + Ti0 2 . 1 The 
weighed precipitate is fused with pyrosulphate, dissolved in 
5 percent sulphuric acid and the titanium determined colori- 
metrieally. 

On account of the variable amounts of carbon in the dif¬ 
ferent samples, the analytical results were computed to a 
metallic basis, and the ratio of the elements determined on 
the basis of 100 parts of metallic zirconium. For the com¬ 
parison of the various electric furnace products a study of 
this ratio is more easily understood than a study of the per¬ 
centage composition of the samples. 


Experimental 

« « 

The zirconium ore employed in this investigation was a 

finely ground zirkite, 65 percent of which passed through a 
100-mesh sieve. Duplicate analyses of ignited samples of 
the ore gave the following composition: 

1 Any iron which escapes the precipitation with hydrogen sulphide will 
appear in this precipitate. Experience has shown, however, that the amount of 
iron which survives a double precipitation with hydrogen sulphide is so small 
that it may be disregarded for all except the most exact determinations. The 
color of this ignited precipitate is a reliable indicator of the composition. A pure 
white color indicates that zirconium alone is present. Titanium produces a 
yellow color, and a reddish or brownish tinge indicates the presence of iron. 



A Preliminary Study of Zirkite Ore 822 


Percent 

Percent 

Zr0 2 . 

72.55 

72.78 

Si0 2 . 

17.34 

17.26 

Fe 2 0 3 . 

4.11 

4.14 

Ti 0 2 . 

0.80 

0.81 

A1 2 0 3 . 

5.28 

5.91 

Rare Earths (cerium group). 

1.04 

1.13 

P 2 O 5 . 

0.49 

0.45 

Mn0 2 . 

0.25 

0.27 

MgO. 

trace 

trace 

Total. 

Loss on ignition. 

101.86 

2.88 

102.75 1 
2.88 


Ratio, on a metallic basis, of the elements present in the ore: 

100 Zr: 15.1 Si: 5.37 Re: 0.89 Ti 

Owing to the time and labor consumed by the necessary 
analytical work, it was decided to limit this investigation 
to the three impurities silicon, iron, and titanium. Since 
silicon is present in far larger amounts than any other impurity, 
this investigation was directed primarily towards the elimina¬ 
tion of silicon. The removal of iron is a secondary object 
of the investigation. 

Two possibilities were considered for the removal of 
silicon and iron in the electric furnace: 

(1) Reduction of the oxides to the metals in the presence 
of an excess of iron, forming ferro-silicon which could be re¬ 
moved subsequently by mechanical means. 

(2) Reduction of the oxides to the metals or carbides 
with subsequent volatilization resulting from the application 
of higher temperatures. 

The first method was tried out in a small crucible furnace 
heated by a direct arc. Iron oxide was added to the charge 
in an amount sufficient to unite with all of the silicon to form 

1 The error in the analyses probably is due to adsorption of alkalis by the 
cupferron precipitate; but may be due in part to the existence in the ore of ele¬ 
ments, e. g., iron and manganese, in different states of oxidation than the ones 
represented in the above table. The composition of the silicates of zirconium, 
which occur in zirkite ore, has not been determined definitely. 





















J G. Thompson 


ferro-silieon containing 50 percent iron. Several runs were 
made but no evidence of the formation of ferro-silicon was 
obtained. In every case the iron and silicon remained dis¬ 
tributed uniformly throughout the charge, probably on account 
of the high viscosity of the melt which prevented the desired 
formation and coalescence of ferro-silicon. Increasing the tem¬ 
perature would increase the fluidity of the melt and a point 
might be reached at which the ferro-silicon would coalesce and 
settle out. The indications are, however, that the desired de¬ 
gree of fluidity would not be reached below the temperature at 
which silicon becomes volatile. If it is necessary to em¬ 
ploy temperatures high enough to cause at least partial volatil¬ 
ization of the impurities, it would seem advisable to rely 
wholly upon volatilization to remove the impurities. The 
latter method also would avoid contaminating the ore by the 
addition of iron, which is necessary if the ferrosilicon scheme is 
followed. Accordingly the attempts to eliminate silicon and 
iron, as ferro-silicon, were abandoned in favor of attempts to 
volatilize the impurities directly. 

Preliminary experiments conducted in a small arc fur¬ 
nace indicated that zirconium carbide is stable at tempera¬ 
tures above the decomposition point of silicon carbide (car¬ 
borundum). 1 This led to the belief that conversion of the 
entire ore to carbides, followed by the exposure of the mixed 
carbides to temperatures above 2220° C, at which tempera¬ 
ture silicon carbide decomposes, would eliminate silicon and 
perhaps some or all of the iron and other impurities. If 
this elimination of impurities were successful, it would then 
be possible to ignite the zirconium carbide to the oxide thus 
producing pure and highly refractory zirconia. 

The first experiments were carried out in a furnace of 
the silicon carbide type, sufficient carbon being added to 
ensure the transformation of all the oxides present into car¬ 
bides. Later experiments were performed in an arc furnace, 
the amount of carbon being varied from run to run. 


1 Gillett: Jour. Phys. Chem., 15, 213 (1911). 



824 


A Preliminary Study of Zirkite Ore 

% 

Experiments with a Resistance Furnace 

The resistance furnace was built in the form of a rectang¬ 
ular trough 7 inches wide at the bottom, 8 inches wide at the 
top, 9 inches deep, and 29 inches long. The floor was built of 
zirkite brick supported by a double layer of fire brick. The 
sides were built of fist-size lumps of zirkite, backed with the 
fire brick and faced smooth with ground zirkite bonded with a 
little water-glass. The sides and floor were permanent. The 
end walls were faced with zirkite brick, which were backed up 
with fire brick, and were torn out after each run to facilitate 
removal of the charge. The electrodes were pieces of 4-inch 
square carbon electrodes which entered through the center of 
the end walls and protruded four inches into the furnace. The 
outer ends of the electrodes were water-cooled and connected 
to bus bars by flexible leads. Power was supplied by a 75 
kilowatt motor-generator set. Two thousand amperes was 
the largest current available, representing the maximum out¬ 
put of the motor-generator set. 

The complete conversion of 100 parts of ore to the various 
carbides and carbon monoxide requires approximately 33 
parts of carbon. To ensure the presence of an excess of carbon, 
a ratio of 45 parts of carbon to 100 parts of ore was used. 
Petroleum coke, 10-20 mesh in fineness, supplied the carbon 
for most of the runs, although in one or two cases granular 
electrode carbon was tried. 

Four runs were made in the resistance furnace. Since the 
runs were all more or less alike, the first one only will be 
described in detail, as follows: 

Run 1 

Ratio of ore to carbon in charge. 100 : 45 

Total weight of charge. 86.5 pounds 

Core. A double line of 

graphite electrode pieces, one inch in diameter, the junctions 
packed in petroleum coke to ensure contact 

Duration of run. 1 hour, 52 minutes 

Power consumption... 60 K.W.H. 








825 


J. G. Thompson 


The charge was packed loosely in the furnace up to the 
level of the electrodes. The core was inserted and the remain¬ 
ing charge added, filling the furnace. At first it required a 
potential of 46 volts to produce a current of 400 amperes 
through the furnace. Small gas volcanoes appeared almost 
at once, followed by flames, bright yellow at the base with 
reddish yellow tips. The power input was maintained as 
high as possible, being limited by the violence of the gas evolu¬ 
tion which resulted in loss of charge when the power input 
became too high. After the first violent evolution of gas 
subsided, the charge presented the appearance of gentle 
boiling. After half an hour, when the total power input had 
reached 15 K.W.H., one end of the charge ceased boiling and 
settled to form a crust. This effect spread gradually until 
the entire charge was crusted over and quiet. Heating was 
continued about half an hour more. The entire charge finally 
became bright red in color but the temperature could not be 
raised further. At first the current was carried almost en¬ 
tirely by the core; but the charge itself soon began to conduct 
with the result that the current rose and the voltage fell off 
rapidly, until at the end of the run the current reached 1750 
amperes and the voltage dropped to 5. These results made 
it evident that the conductivity of the core and heated charge 
was too great to allow the input of power at a rate sufficient 
to produce the very high temperature desired. 

When the cooled charge was examined, a grayish black 
granular core about 4 inches in diameter was found in the 
center, extending the length of the charge between the elec¬ 
trodes. Small deposits of spongy material, metallic in ap¬ 
pearance and rather bluish in color, which subsequent exami¬ 
nation proved to be zirconium carbide, were found in close 
proximity to the original core. The total weight of “sponge” 
was about six pounds. The remaining contents of the furnace 
consisted of unchanged charge. The original graphite core 
was not attacked, indicating the presence of an adequate 
supply of carbon in the charge. Some slight indications of 


A Preliminary Study of Zirkite Ore 


826 


the volatilization of silica were found on the furnace walls, 
but the power input evidently was too low. 

The three subsequent runs were made in an attempt to 
increase the power input, but without marked success. The 
results of the four runs may be summarized as follows: 


Tx\ble I 


K. W.H. 

Av. Kw. 

Carbide 

“sponge” pounds 

Metal in Sponge when Zr = 100 

Si 

Fe 

Ti 

60 

32 

6.0 

3.7 

4.4S 

0.79 

75 

34 

6.5 

11.0 

5.50 

0.78 

100 

38 

10.0 

12.2 

7.60 

0.79 

80 

40 

7.5 

5.5 

2.60 

0.87 



Original Ore 

15.1 

5.37 

0.89 


The behavior of the carbide “sponge” on analysis showed 
that most of the silicon was present as the carbide, thus 
indicating that in general the temperature of the mass was not 
high enough to ensure the removal of the silicon by volatil¬ 
ization. As the data indicate, the removal of silicon varied 
in a very irregular manner, being fairly complete in some 
cases but very incomplete in others. Other samples from 
portions of the charge farther removed from the core showed 
higher ratios of silicon to zirconium. This follows naturally, 
since volatilization of silicon would occur first in the hottest 
portion of the charge, i. e., around the core, and would be 
less evident in regions at a greater distance from the source 
of heat. 

The data indicate little or no elimination of titanium 
or of iron, except in the fourth run where considerable iron 
was apparently removed. In this run, however, about 9 
pounds of common salt was added to the charge resulting in 
the elimination of approximately half the iron, presumably 
through the formation of the volatile chloride. All things 
considered, the results obtained with the resistance furnace 
were unsatisfactory and this type of furnace was abandoned 
in favor of one of the arc type. 
















827 


J. G. Thompson 


Experiments with an Are Furnace 

The furnace shell of the arc furnace consisted of an iron 
pot, lined with fist-sized pieces of zirkite ore. The lining 
was faced smooth with ground zirkite bonded with a little 
water glass, and contained the same ratio of carbon to zirkite 
as the charge proper. The resulting hearth was cone-shaped, 
8 inches in diameter at the top, 4 inches in diameter at the 
bottom, and 8-10 inches deep. This lining was torn out 
after each run to facilitate removal of the charge. The 
lower, horizontal electrode, composed of two 2-inch square 
graphite electrodes, formed the floor of the hearth, enter¬ 
ing through a rectangular opening in the side of the cast iron 
pot. The upper electrode was a 4-inch square carbon electrode, 
suspended in a vertical position and counter-weighted to allow 
adjustment. Both electrodes were clamped in water-cooled 
electrode holders and connected to bus bars by means of 
flexible cables. 

The first run performed in this furnace will be described 
in detail, as being typical of all the runs carried out subse¬ 
quently. 

Run 5 


Ratio of ore to carbon in charge 

Total weight of charge..... 

Time of run. 

Power consumption. 


100 : 45 
11 pounds 
45 minutes 
32. 5 K.W.H. 


The run was started by striking an arc between the bare 
electrodes. . The walls of the cone immediately began to slag 
down so that the first addition of charge was made as soon as 
possible after starting the arc. The charge was fed in slowly 
and was observed to solidify when it reached the zone of the 
arc. During the early stages of the run the arc was smothered 
by the charge but after all the charge had been added the arc 
was run open, and remained fairly quiet. The current was 
maintained at about 1000 amperes until the evolution of 
fumes had almost ceased when the arc became noisy and hard to 
maintain. The run was stopped when this point was reached. 

The top of the charge oxidized during cooling, forming 









A Preliminary Study of Zirkite Ore 


828 


a layer of fine white oxides, below which lay a brittle, black 
mass weighing approximately 3.5 pounds. A sample for 
analysis was taken from the center of this cake and gave the 
following ratio: 

100 Zr: 4.85 Si: 6.38 Fe: 0.70 Ti 

A series of runs was made in which the ratio of carbon 
(petroleum coke) to zirkite ore was varied systematically. 
The results are assembled in the following tables. 


Table II 

Effect of Varying Ratio of Ore to Carbon 


Run 

Charge Pounds 

Carbon 
per 100 ore 

K.W.H. 

Yield purified 
product pounds 

5 

11.0 

45 

32.5 

3.5 

9 

12.5 

25 

18.25 

3.0 

10 

12.0 

20 

17.5 

3.1 

11 

15.4 

15 

21.5 

5.0 

18 

15.4 

10 

19.3 

7.0 

19 

16.8 

5 

27.0 

9.9 


Metal Ratio in Product 


Run 

Zr 

Si 

Fe 

Ti 

5 

100 

4.85 

6.38 

0.70 

9 

100 

3.03 

6.50 

0.64 

10 

100 

2.21 

5.00 

0.55 

11 

100 

0.88 

5.92 

0.70 

18 

100 

1.31 

8.73 

0.69 

19 

100 

1.86 

6.67 

0.73 

Ore 

100 

15.70 

5.37 

0.89 


Table III 

Elimination of Impurities 


Run 

Carbon per 100 ore 

Percentage elimination of 

Si 

Fe 

Ti 

5 

45 

68 

-19 

21 

9 

25 

80 

-21 

28 

10 

20 

85 

7 

38 

11 

15 

94 

-10 

21 

18 

10 

91 

-62 

22 

19 

5 

88 

-24 

18 





































829 


J. G. Thompson 


The data obtained with the arc furnace show a maximum 
in the removal of silicon when the ratio of zirkite to carbon 
is 100:15. Calculations based on the silica content of the 
ore show that a little more than 10 parts of carbon to 100 
parts of ore is just enough to transform all of the silica into 
carbide (carborundum) and carbon monoxide. We have, 
therefore, come to the conclusion that the best high-tempera¬ 
ture elimination of silicon is obtained when little more than 
enough carbon is used to form carbide with the silicon only, 
leaving the zirconium oxide undecomposed. 

When more than 15 parts of carbon are used the elimina¬ 
tion of silicon becomes steadily less complete. Under these 
circumstances it is known that zirconium carbide is actually 
produced in the arc. The zirconium carbide therefore appears 
to retain some of the silicon and the suggestion is offered that 
stable double carbides of silicon and zirconium may be pro¬ 
duced or that the two carbides form solid solutions or mixed 
crystals. Runs in which granular electrode carbon was sub¬ 
stituted for petroleum coke showed that better results may be 
obtained on a small scale with the more finely divided coke. 

An interesting fact brought out by these experiments is 
the high melting point and stability, in a reducing atmosphere, 
of the carbide of zirconium. As long as there was an excess 
of carbon present, over and above the amount required for the 
complete conversion of the oxides to carbides, it was impossible 
to melt the product even under the direct action of a 40-50 
killowatt arc. 

The partially purified carbide apparently had a higher 
melting point than the oxide product with the same relative 
composition. This was shown in the runs in which the ore 
melted and formed a pool under the arc, the pool solidifying 
as soon as more carbon was added. In order to use the carbide 
as a refractory, however, it would be necessary to protect it 
from oxidation. The black carbides all burned readily in the 
air to form light, fluffy oxides. Heating the powdered car¬ 
bides to dull redness is sufficient to start the oxidation which 
then proceeds slowly but persistently. 


A Preliminary Study of Zirkite Ore 830 

0 

The data show that some titanium was also eliminated but 
no iron, there being an apparent increase in the amount of 
the last impurity. This apparent increase in the iron content, 
especially in the case of Run 18, is due to the difficulties 
encountered in analysis. The samples of furnace products 
must be pulverized for analysis and, since the amount of 
silicon present is of primary importance, grinding such abrasive 
material in an agate mortar is out of the question. The only 
other method available was the use of a cast-steel bucking 
board, which resulted, naturally, in contamination of the 
samples. In view of the time required for analysis it was not 
considered advisable to grind additional samples in an agate 
mortar for the determination of iron. From a consideration 
of the data in Table III, therefore, one may safely conclude 
only that little if any iron was eliminated under the conditions 
of experiment. 

Other runs were made in which sodium chloride was added 
to the charge of ore and coke to aid in the removal of iron, 
but curiously enough no iron was eliminated in this way. It 
is possible that the rapid attainment of the high temperature 
under the arc decomposed the chloride before it could be 
volatilized away. 

Since the treatment in the electric furnace was not suc¬ 
cessful in eliminating the iron, other means of accomplishing 
this end were sought. 

Experiments on the Removal of Iron 

Different samples of the various electric furnace products 
all showed magnetic properties, due presumably to the pres¬ 
ence of iron either as carbide or alloy. Accordingly attempts 
were made to use these magnetic properties as a basis of sepa¬ 
ration of the iron but the attempts were not successful. The 
magnetic portions contained approximately half of the zir¬ 
conium and the non-magnetic portions contained appreciable 
amounts of iron. 

Attempts were next made to remove the iron by leaching 
with 5 percent sulphuric acid. 1 Both the carbide and the 


1 Barton: loc. cit. 



831 


J. G. Thompson 


corresponding oxidized products of the electric furnace runs 
were leached with hot and with cold 5 percent sulphuric 
acid. The iron in the carbide appears to be more susceptible 
to leaching than is the iron in the oxidized material; but, 
even in the case of the carbide, less than half of the iron could 
be removed in this way. 

A series of experiments was next carried out on the re¬ 
moval of iron by treatment of the carbides or mixed oxides 
with chlorine 1 at both low and high temperatures. The ex¬ 
periments were carried out in a small, horizontal, tube fur¬ 
nace and the frequent clogging of the apparatus by the vola¬ 
tile products of the reactions, especially at high temperatures, 
showed plainly that a furnace of special design is essential for 
investigation of this possibility. Nevertheless, results were ob¬ 
tained which indicated that iron may be removed in this way 
from zirconium carbide at low temperatures and, at high tem¬ 
peratures, from zirconium oxide from which silicon had been 
largely eliminated in the arc furnace. It would seem, therefore, 
that a fairly pure zirconia for refractory purposes might be 
made from crude zirkite ore by eliminating in an electric furnace 
as much as possible of the silicon and by following this by 
treatment wfith chlorine or phosgene 2 to remove the iron. 

This investigation is admittedly incomplete in many ways. 
The limited time at our disposal and the difficulty of, and 
the time consumed by, the analytical work involved, may be 
offered as an excuse. It is hoped, however, that attention 
will be called to the problem of large-scale purification of 
zirconium oxide, so that the latter may become more generally 
available as a refractory material for very high temperatures. 
A summary of the results obtained follows: 

Summary 

(1) Ninety to ninety-five percent of the silicon may 
be removed from siliceous zirkite ore by heating a mixture 


i 


Phillips: loc. cit. 

Baskerville: Science, 50, 443 (1919). 



A Preliminary Study of Zirkile Ore 


832 


of ore and carbon to a temperature greater than 2220° C in an 
electric furnace. 

(2) The best results appear to be obtained by feeding 
into an arc furnace a mixture of ore and coke, the amount of 
carbon being approximately that required to transform only 
the silicon to the carbide. 

(3) The existence of stable double carbides of silicon 
and zirconium or of solutions of silicon carbide in solid zir¬ 
conium carbide has been suggested as an explanation of the 
incomplete removal of silicon when carbon in excess of that 
required to form only silicon carbide is used. 

(4) It is suggested that zirconia sufficiently pure for 
refractory purposes might be obtained from zirkite ore by 
removing the silicon in an electric furnace and following this 
treatment with chlorine or phosgene to remove the iron. 

(5) Attention has been called to the refractory properties 
of zirconium carbide and the factors which limit its use. 

(6) Methods of analysis of zirconium compounds have 
been reviewed briefly and a modified scheme of analysis has 
been outlined for the determination of zirconium and the three 
major impurities, silicon, iron, and titanium. 

The author acknowledges his indebtedness to Professors 
Bancroft and Briggs for direction and advice in the pursuit 
of this investigation. 


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